Visual Studio 2008 performance profiler problems with ASP.NET

I've been trying, and failing, to get VS.NET 2008 to profile an ASP.NET MVC application. When I try and start a profiling session, I get a generic server 500 error from VS.NET:

The web site could not be configured correctly; getting ASP.NET process information failed. Requesting 'http://localhost:4750/foo/VSEnterpriseHelper.axd' returned an error: The remote server returned an error: (500) Internal Server Error.

I've tried several things, including:

• ensuring that web.config is writable
• ensuring that there aren't any permissions issues (everything is running as me, I'm a system administrator, and UAC is off)
• everything mentioned here: 500 Error Running Visual Studio ASP.NET Unit Test

A brand new ASP.NET MVC application (i.e., File | New | Project) will profile without problem, so I know my machine is capable of profiling such an application.

What are the other common and uncommon things that can prevent VS.NET from starting a profiling session on an ASP.NET application?

If you can't get VS.NET profiling to work, there are a couple of third-party tools that might help. Obviously you’ll need to pay for these though.

ANTS has a free demo here.

JetBrains also have a trace tool that offers profiling (I have not used this, but I like their ReSharper tool, so I suspect this will be good).